1. Field of the Invention
The present invention pertains to matrix display screens, and more particularly to an active matrix structure of pixel elements offering both large storage capacity on each pixel electrode and a high aperture ratio. The field of the invention is that of active matrices formed on a transparent substrate.
2. Discussion of the Background
A matrix display screen is customarily formed of two transparent substrates, for example made of glass, facing one another, joined together by a peripheral seal so as to allow a cavity in which is located the display material such as liquid crystals or light-emitting diodes made of organic material (OLEDs as they are known). The first substrate is clad with a back electrode, common to the whole matrix, linked to a reference potential. In the case of a color screen, this substrate furthermore comprises a matrix of colored filters (typically, red, green and blue filters). The second substrate comprises the pixel electrodes, arranged in matrix fashion in rows and columns. In the case of active matrices, the second substrate also comprises switching devices such as a transistor or a diode, with a device for each pixel electrode. Each switching device is connected to a row and to a column of the matrix through which a display data item may be applied selectively (matrix addressing) to the corresponding pixel electrode.
The structure formed by the pixel element, the portion of back electrode opposite and the dielectric between, defines a pixel capacitor. This pixel capacitor has a small capacitance. For this reason, a separate storage capacitor is provided in the structure, associated with each pixel electrode, so as to maintain the voltage level applied to the pixel electrode by the associated switching device, the whole time of each scan frame. According to the voltage level applied to the pixel electrode, with respect to the reference level of the back electrode, a white (or green, red or blue, in the case of a screen with color filters) image dot or a black image dot is obtained at the corresponding location on the screen.
The refreshing of the images on the screen is obtained according to a frame scan mode: the rows of pixel elements are selected in turn by means of a scan signal applied successively to each of the lines for selecting rows of the matrix and the display data corresponding to each row are applied to the columns. The application of the frame scan signal to a line has the effect of turning on the switching devices of the associated row. Each of these devices then switches the display data item applied to the associated column, typically a voltage level, onto the associated pixel electrode. The voltage level on the pixel electrode must be maintained without losses, the whole time of the frame. Now, the leakage currents of transistors in the off state, and the stray capacitances with the lines and the columns are so many factors that bring about a discharge of the pixel capacitor. It has been seen that a separate storage capacitor with a suitable value was thus provided, associated with the pixel electrode, for this function, with the aim of obtaining as good as possible an image contrast and of reducing the phenomena of flicker.
According to a state of the art as described for example in the article entitled “Driving method for gate-delay compensation of TFT/LCD” by K. Kusafuka, H. Schimizu, S. Kimura, published in IBM J. Res. Develop. Vol. 42 No. 3/4 May/July 1998, a storage capacitor is customarily produced between the pixel electrode and the selection line of the previous row of pixels. In a matrix with switching elements of transistor type, the selection line produces the gate of the transistors of the row, hence the name “gate storage capacitor” (or capacitor on gate as it is known in the literature), for this storage capacitor produced with this selection line. However, with such storage capacitor structures, the metal of the selection line, which metal is opaque, encroaches onto the surface of the pixel electrode. This have the effect of decreasing the aperture ratio OAR of the pixels (open aperture ratio). In practice, a compromise must be made between the storage capacitor value that one seeks to obtain and the degradation of the aperture ratio of the pixel elements that would be acceptable. This solution turns out to be poorly suited for so-called high resolution screens, for which the pixel elements are already of small dimensions.
According to another state of the art, the storage capacitor may be produced by a ground plane buried under the matrix of pixel elements, such as for example described in European patent no. 0 607 352 (92 922674.4). A layer of conducting and transparent material, such as indium tin oxide (ITO) is thus produced over the whole surface of the substrate, preferably above a first layer of conducting or nonconducting opaque material, which forms a screen (light shield). The template of this opaque layer is produced so as to mask all the parts outside the useful aperture zone of the pixels, typically the selection lines and the data lines, so as to screen the field lines of the planar stray capacitances (stray capacitances between the pixel electrode and the lines and the columns).
The structure of the active matrix comprising the switching elements, typically transistors or diodes, and the pixel electrodes, is thereafter produced according to the customary technological processes.
The storage capacitor of each pixel element is then produced to 90% by the capacitive structure between the pixel electrode and the portion of ground plane buried opposite, and to 10% by the portion of selection line of the previous row opposite the pixel electrode.
The buried ground plane is connected to a reference potential, typically the potential of the back electrode, by means of an external contact pad.
For certain applications of active matrix screens, one seeks however to reduce the constraints of external connection. The need to connect the ground plane to a reference potential then appears as a constraint. Specifically, problems of crossover with the other external signals arise, in particular the scanning signals, thereby rendering the design of the installation scheme more complex.
Finally, despite the insulation layers provided in the structure of the matrix (generally three) between the various conducting levels, and according to the fabrication technologies used, in practice short-circuits may occur between the ground plane and the various conducting levels of the structure, due to dust or impurities: short-circuits with the selection lines or the pixel electrode. The pixel electrodes concerned are then short-circuited to the potential of the back electrode: these pixels therefore appear white (or else red, green or blue, in the case of a color screen) These visible defects affect the fabrication yield.